1. Field of the Invention
The present invention relates to a construction of an optical disc and a lens in an optical disc device.
2. Description of the Related Art
Optical disc devices converge a laser beam on an optical disc by a lens to read and write information. These devices employ a servo control for focusing which involves reading a focus deviation from a reflected beam and feeding it back to drive an actuator for the lens so that a spot of the converged laser beam is focused on a recording surface of the optical disc.
As the recording density of optical discs has increased in recent years, a reduction in a diameter of laser spot has been under study. The study includes using a lens of high numerical aperture, which tends to shorten a distance between the lens and the optical disc.
It is therefore feared that when the device is impacted, the lens and the optical disc may easily strike each other.
As a means to avoid collision between the lens and the optical disc, a method has been proposed which, when an impact detected by an impact sensor exceeds a predetermined value, cancels a focus servo and applies a force to the lens causing it to move away from the optical disc (JP-A-2003-233911).